In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time.
Turbulence causes the formation of eddies of many different length scales. Most of the kinetic energy of the turbulent motion is contained in the large-scale structures. The energy “cascades” from these large scale structures to smaller scale structures by an inertial and essentially inviscid mechanism. This process continues creating smaller and smaller structures that produces a hierarchy of eddies. Eventually this process creates structures that are small enough that molecular diffusion becomes important and viscous dissipation of energy finally takes place. The scale at which this happens is the Kolmogorov length scale.
The interaction of the energetic eddies with the walls of a fluid dynamic structure creates broadband noise. For example, eddies from the turbulent boundary layer flow scatter are a significant source of noise when traveling past the trailing edge of airfoils. When a lifting airfoil moves through a turbulent stream, an unsteady pressure field develops around the airfoil, which produces acoustic waves that manifest into broadband noise. Such phenomena can readily be identified in nearly all types of vehicles, for example, automobiles, planes, helicopters, ships, submarines, and the like, the results of which can be quite troublesome. For example, helicopter noise levels can easily approach 100 dB with a large portion of this noise related to the rotor blades cutting through the wake of the leading blade. Sophisticated submarine propulsors, which are similar in form to an axial compressor in a turbofan engine, can produce considerable noise as the propulsor blades slice through boundary layer produced turbulence. In combat scenarios, such noise levels obviously hamper the stealth capability of these vehicles. In addition, with the passage of Bills, such as the “Silent Skies Act of 1999”, industry is increasingly coming under pressure to reduce the levels of noise pollution associated with turbomachinery propulsors (i.e. turbofans, turbojets, etc) on their aircraft. Similarly wind turbine noise limits their public acceptance as well as the growth of the rotor to extract more renewable energy from the earth's wind resources; boundary layer noise, i.e, the interaction of turbulence in the boundary layers of the wind turbine blades with the airfoil surface, is one of the primary sources of wind turbine noise. Thus, there is a wide-spread need to reduce noise produced by the interaction of eddies with the fluid dynamic structure.